Abstract
The methanol to gasoline process over the zeolite catalyst ZSM-5 in a lab-sized reactor bed (4 mm diameter) has been studied in operando with high energy synchrotron X-ray diffraction. The fast z-scan method was used, scanning the reactor repeatedly and at speed through the X-ray beam. The X-ray diffraction data were processed using high throughput parametric Rietveld refinement to obtain real structural parameters. The diffraction data show only very subtle changes during the process and this allows us to demonstrate the combination of very large data volumes with parametric Rietveld methods to study weak features of the data. The different possible data treatment methodologies are discussed in detail and their effects on the results obtained are demonstrated. The trends in unit cell volume, zeolite channel occupancy and crystallite strain indicate that more or larger reaction intermediates are present close to the reactor outlet.
Highlights
The example used is the methanol to gasoline (MTG) conversion process over the highly crystalline aluminosilicate zeolite catalyst ZSM-5.35 This catalyst has been used for conversion of methanol to fuels since
The data on the MTG were analysed by a parametric method using the structural model of as synthesised ZSM-5 reported by Van Koningsveld et al.[83]
The lattice parameters of the structure were refined for each pattern as restricted by symmetry and the broadening of the peaks in each pattern was modelled with the TOPAS Lorentzian strain broadening macro, on the assumption that strain in the crystallites rather than particle size would be influenced by any changes caused by the build-up of reaction intermediates inside the zeolite catalyst during reaction as for SAPO-34 in the methanol to olefin (MTO) process[4,6]
Summary
Over the last decade X-ray diffraction studies of chemical reactions have moved from being a relatively niche area of science providing limited information on areas such as zeolite synthesis to a much more generally applicable method.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21] Recent advances in high energy X-ray detectors[22] and Rietveld processing methods[23] allow us to study reactions with excellent time resolution and even space resolution, while obtaining real crystal structure data.[3,4,5,6,24,25,26,27,28] It is even possible to obtain three-dimensional tomographic data for working catalysts at a speed comparable to that of some industrial catalytic reactions.[29,30,31,32]. The high crystallinity of the zeolite catalyst and the large body of existing research on adsorption of organic molecules makes X-ray diffraction a very suitable method for studying the MTG process over ZSM-5. Data for validation of the behaviour of the catalyst were collected on beamline BM01A of the ESRF (The Swiss Norwegian beamline) using the capillary reactor described by Norby et al and used in numerous PXRD studies of catalysts and other systems.[3,4,5,18,19,20,21,78,79,80] Helium bubbled through methanol at 20 1C was fed over the catalyst at a rate of 20 ml minÀ1. A typical TOPAS input file for the parametric refinement is included as ESI.† Curve fitting was carried out in Origin 8.6
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